This invention relates to a method for rounding the edges of parts, in particular of turbo engines.
It may be necessary, for various reasons, to round the edges of parts, in particular on turbo engines. These include improving the strength and/or aerodynamics and preventing the risk of injury. Depending on the part, there may be sharp edges on parts that are to be rounded to the adjacent surfaces of the part. Alternatively, the edges may also form planar or three-dimensional surfaces which connect adjacent surfaces of the part, usually much larger surfaces. The latter case usually occurs with relatively coarsely prefabricated edges on hydromechanically active blades of turbo engines, in particular on the guide vanes and rotor blades of gas turbines, where the blade edges with the adjacent pressure side and/or suction side of the blades must be rounded for reasons of fatigue, strength and aerodynamics.
It is known that surfaces must be roughened by abrasive blasting before coating operations to clean the surfaces and improve adhesion to the layer. German Patent Document No. DE 697 12 613 T2 additionally discloses a method for honing cutting edges, whereby these edges are machined by abrasive fluid jets using the abrasiveness to introduce fine grooves into the surface.
German Patent Document No. DE 197 20 756 C1 discloses a method for surface treatment wherein the surface is subjected to a particle bombardment. This introduces compressive stresses into the material to increase the long-term strength and the tensile strength of the part in particular.
In the case of blade edges, which are generally premachined only relatively coarsely due to the manufacturing technique, rounding has so far been performed largely by manual labor, using hand-guided machines such as belt grinders, etc., if necessary. This is very labor-intensive and time-consuming and ultimately uniform, reproducible machining results cannot be guaranteed even with targeted control and testing.
In view of these known methods and their disadvantages and their limits in terms of applications, the object of the present invention is to provide a method for rounding edges, which permits a great savings of time and personnel and leads to reproducible results through machine operation, optionally automatable. These reproducible results should be of the highest possible quality, achievable in a satisfactory manner with the lowest possible reject rate.
It has surprisingly been found that by abrasive blasting, taking into account defined machining parameters and nozzle definitions, relatively accurate rounded surface geometries can be produced on sharp edges of parts or relatively coarsely premachined blade edges. The functional reliability of this method and its reproducibility have been confirmed in experiments.
In the inventive method, the blasting jet is adjusted with its center approximately tangential to the angle bisecting line on the edge between the (generally) two surfaces on which the rounding is to be performed. In the case of surfaces meeting in the form of a sharp edge, the position of the angle bisecting line is immediately obvious. In the case of surfaces that do not meet directly, e.g., are joined by an edge in the form of a planar or three-dimensional surface, such as the pressure side and the suction side of a coarsely prefabricated edge of the blade of a gas turbine, tangents are drawn to the two surfaces at such an edge and the angle bisecting line between the intersecting tangents is determined. In the latter case of an edge to be rounded, the edge adjoining the pressure side and suction sides of a blade, this angle bisecting line is tangent to the center line of the profile of the blade at the edge, i.e., at the stagnation point.
Relatively small particles with a size of 0 to 500 mesh, preferably 180 to 320 mesh, are used to reduce any remachining of the rounded edges. In this way, abrasion of material for rounding is created by this method and cracks and roughness on the surfaces are prevented.
To create a blasting jet having a defined geometry and energy with regard to cross section, shape, etc., the blasting jet is produced by a nozzle having a defined outlet diameter and a defined outlet angle.
To produce a uniform geometry along the edge, the relative movement between the nozzle and the part may preferably take place in a defined variable distance between the nozzle and the blade edge.
The distance is generally adjusted continuously in a suitable manner, in the case of large-area edges with a width that changes over their length.
The direction of the center of the blasting jet to the center line of the profile of the blade at the edge of the blade may be set at an angle β and/or may be laterally offset in relation to the center line of the profile in the direction of the pressure side or suction side to create, for example, aerodynamically desirable contour symmetries on the edge to be rounded.